EVIDENCE OF PREHISTORIC DEBRIS-FLOW ACTIVITY IN THE MASSANUTTEN MOUNTAINS, VIRGINIA

Extensive deposits of angular blocks and/or sub-angular to sub-rounded boulders of Massanutten Sandstone have been identified in the vicinity of the Massanutten Mountains in Rockingham, Page, and Shenandoah Counties, northwestern Virginia during recent 1:24,000-scale geologic mapping of the Elkton West and Tenth Legion 7.5-minute quadrangles. Where observed, these deposits generally lack sorting and imbrication and are supported by a sandy loam matrix. Four main types of deposits are recognized: 1) linear deposits that are most common in dip and antidip stream valleys that drain mountain ridges; 2) terraces that are most common in strike stream valleys between mountain ridges; 3) fans on lower slopes and on the valley floor that are best developed adjacent to major water gaps; and 4) caps and remnant deposits on ridges adjacent to dip and antidip streams and isolated hilltops on lower slopes and the valley floor. Based on the extreme size of the largest clasts in some of these deposits, a lack of alluvial bedding features, and a similarity in map pattern to modern debris-flow deposits in the Appalachians, we interpret these deposits to be the result of prehistoric debris flow activity. The deposits can be grouped into older deposits that are dissected and form topographic highs, intermediate-age deposits that are not dissected but have streams or erosion scarps developed along their edges, and younger deposits that occur along or adjacent to active drainages. Multiple elevations of older surfaces, distinct erosion scarps within younger deposits, and deposits away from active drainages suggest that many debris-flow events have occurred in this area over a long period of time and that debris-flow events cause changes in surface drainage patterns. The extensive distribution of these deposits suggests that debris flows have played a major role in denudation and landscape evolution of the Massanutten Mountains.